powerpc/64: Fix kernel stack 16-byte alignment

Commit 4c2de74cc869 ("powerpc/64: Interrupts save PPR on stack rather
than thread_struct") changed sizeof(struct pt_regs) % 16 from 0 to 8,
which causes the interrupt frame allocation on kernel entry to put the
kernel stack out of alignment.

Quadword (16-byte) alignment for the stack is required by both the
64-bit v1 ABI (v1.9 § 3.2.2) and the 64-bit v2 ABI (v1.1 § 2.2.2.1).

Add a pad field to fix alignment, and add a BUILD_BUG_ON to catch this
in future.

Fixes: 4c2de74cc869 ("powerpc/64: Interrupts save PPR on stack rather than thread_struct")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/ptrace.h

@@ -54,6 +54,7 @@ struct pt_regs
 
 #ifdef CONFIG_PPC64
 	unsigned long ppr;
+	unsigned long __pad;	/* Maintain 16 byte interrupt stack alignment */
 #endif
 };
 #endif

arch/powerpc/kernel/setup_64.c

@@ -636,6 +636,8 @@ static void *__init alloc_stack(unsigned long limit, int cpu)
 {
 	unsigned long pa;
 
+	BUILD_BUG_ON(STACK_INT_FRAME_SIZE % 16);
+
 	pa = memblock_alloc_base_nid(THREAD_SIZE, THREAD_SIZE, limit,
 					early_cpu_to_node(cpu), MEMBLOCK_NONE);
 	if (!pa) {